The subject matter of the present invention is a method for the preparation of partial esters and ortho esters of titanium, zirconium or hafnium by esterifying the corresponding metal halides. The term, "partial esters of titanium, zirconium or hafnium," which are also referred to herein as metal acid partial esters or halogen partial esters, is to be understood to refer to compounds of the formula (RO).sub.n MeX.sub.4-n, in which R can represent alkyl moieties of 1 to 18 carbon atoms, preferably 2 to 8 carbon atoms, or aryl moieties, X represents halogen, preferably chlorine, and n can assume values between 1 and 3.
It is known to prepare ortho esters, especially of titanium and zirconium, on a large technical scale exclusively by reacting the corresponding tetrachlorides with desired organic hydroxy compounds in the presence of amines as acid acceptors and solvents as diluents (cf. J. Nelles, U.S. Pat. No. 2,187,821; CA 34 3764).
A significant drawback of this process is the large amount of amine hydrochlorides produced as waste. The titanium and zirconium ortho esters made in this manner contain considerable amounts of polymers in addition to the usually present solvent and alcohol or phenol moieties. The polymers result from the secondary reaction occurring mainly in the first two stages of esterification with alcohols and resulting in the formation of alkyl halide which simultaneously forms water and which hydrolyzes the metal esters with the simultaneous formation of quality-degrading metal oxane.
Direct esterification, i.e., the reaction of the titanium or zirconium tetrahalides with organic hydroxy compounds with the release of gaseous hydrogen halide, is known to be very slow. Orthoesters, in the absence of amines, can be obtained in this manner only from phenols used in massive excess. The excess phenol, however, is stubbornly retained by the product, so that these products cannot be used as pure ortho esters. On the other hand, direct esterification using aliphatic alcohols does not lead to the ortho ester, but runs slowly as far as the dihalogen diester ("half ester"), and then stops, even with an alcohol excess. The monohalotriesters, which are sought after on account of their stereoselective catalyst properties, are not accessible at all by direct esterification. The very special disadvantage of this known process is also, in the case of aliphatic esters, the pronounced tendency towards the secondary reaction of alkyl halide and water formation and the formation of polymers.
The partial esters of titanium and zirconium, especially the half esters and the triester monohalides, are of considerable interest especially on account of their outstanding catalytic properties in the production of tactic polymers on Ziegler-Natta catalysts, and in stereoselective syntheses in organic chemistry. The attempt has therefore already been made to prepare these partial esters by mixing the ortho esters with the tetrahalides in a controlled equivalent ratio with the intention of obtaining the desired partial ester structure in the partial ester mixture that forms in this process, and which of course contains all stages of esterification in addition to the foreign substances and polymers brought in from the ortho esters, obtaining them at least in an appreciable percentage if not in pure form. For lack of purer products, such partial ester mixtures are presently used as catalyst components with somewhat useful results. However, what is involved is not pure partial esters having a clearly assured structure.
The state of the art thus described is unsatisfactory insofar as, if pure partial esters can be used in the above-described applications, a decided advance could be expected with regard to stereoselectivity or even stereospecificity and tacticity.
Accordingly, the problem was to find an effective method for a highly selective production of pure halogen partial esters and ortho esters of titanium and zirconium, in which the production of ammonium halide will be considerably reduced or completely eliminated.